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Domestic pig

The domestic pig (Sus scrofa domesticus or only Sus domesticus), often called swine, hog, or simply pig when there is no need to distinguish it from other pigs, is a large, even-toed ungulate. It is variously considered a subspecies of the wild boar or a distinct species. The domestic pig's head-plus-body-length ranges from 0.9 to 1.8 m (35 to 71 in), and the adult can weigh between 50 and 350 kg (110 and 770 lb). Compared to other artiodactyls, its head is relatively long, pointed, and free of warts. Even-toed ungulates are generally herbivorous, but the domestic pig is an omnivore, like its wild relative.

When used as livestock, domestic pigs are farmed primarily for the consumption of their meat, called pork. The animal's bones, hide, and bristles are also used in commercial products. Domestic pigs, especially miniature breeds, are kept as pets.

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The domestic pig typically has a large head, with a long snout which is strengthened by a special prenasal bone and a disk of cartilage at the tip.[2] The snout is used to dig into the soil to find food, and is a very acute sense organ. The dental formula of adult pigs is 3.1.4.33.1.4.3, giving a total of 44 teeth. The rear teeth are adapted for crushing. In the male the canine teeth can form tusks, which grow continuously and are sharpened by constantly being ground against each other.[2]

Four hoofed toes are on each foot, with the two larger central toes bearing most of the weight, but the outer two also being used in soft ground.[3]

Pigs possess both apocrine and eccrine sweat glands, although the latter appear limited to the snout and dorsonasal areas.[5] Pigs, however, like other "hairless" mammals (e.g. elephants, rhinos, and mole-rats), do not use thermal sweat glands in cooling.[6] Pigs are also less able than many other mammals to dissipate heat from wet mucous membranes in the mouth through panting. Their thermoneutral zone is 16 to 22°C.[7] At higher temperatures, pigs lose heat by wallowing in mud or water via evaporative cooling; although it has been suggested that wallowing may serve other functions, such as protection from sunburn, ecto-parasite control, and scent-marking.[8]

Archaeological evidence suggests that pigs were domesticated from wild boar as early as 13,000–12,700 BC[citation needed] in the Near East in the Tigris Basin,[15][page needed]Çayönü, Cafer Höyük, Nevalı Çori[16] being managed in the wild in a way similar to the way they are managed by some modern New Guineans.[17] Remains of pigs have been dated to earlier than 11,400 BC in Cyprus. Those animals must have been introduced from the mainland, which suggests domestication in the adjacent mainland by then.[18] There was also a separate domestication in China which took place about 8000 years ago.[19][20]

DNA evidence from subfossil remains of teeth and jawbones of Neolithic pigs shows that the first domestic pigs in Europe had been brought from the Near East. This stimulated the domestication of local European wild boar, resulting in a third domestication event with the Near Eastern genes dying out in European pig stock. Modern domesticated pigs have involved complex exchanges, with European domesticated lines being exported, in turn, to the ancient Near East.[21][22] Historical records indicate that Asian pigs were introduced into Europe during the 18th and early 19th centuries.[19]

In August 2015, a study looked at over 100 pig genome sequences to ascertain their process of domestication, which was assumed to have been initiated by humans, involved few individuals, and relied on reproductive isolation between wild and domestic forms. The study found that the assumption of reproductive isolation with population bottlenecks was not supported. The study indicated that pigs were domesticated separately in Western Asia and China, with Western Asian pigs introduced into Europe, where they crossed with wild boar. A model that fitted the data included a mixture with a now extinct ghost population of wild pigs during the Pleistocene. The study also found that despite back-crossing with wild pigs, the genomes of domestic pigs have strong signatures of selection at DNA loci that affect behavior and morphology. The study concluded that human selection for domestic traits likely counteracted the homogenizing effect of gene flow from wild boars and created domestication islands in the genome. The same process may also apply to other domesticated animals.[23][24]

The adaptable nature and omnivorous diet of the wild boar allowed early humans to domesticate it readily. Pigs were mostly used for food, but early civilizations also used the pigs' hides for shields, bones for tools and weapons, and bristles for brushes.[25] In India, pigs have been domesticated for a long time, mostly in Goa and some rural areas, for pig toilets. Though ecologically logical as well as economical, pig toilets are waning in popularity as use of septic tanks and sewage systems is increasing in rural areas.

Female pigs reach sexual maturity at 3–12 months of age, and come into estrus every 18–24 days if they are not successfully bred. The gestation period averages 112–120 days.[41] Estrus lasts two to three days, and the female's displayed receptiveness to mate is known as standing heat. Standing heat is a reflexive response that is stimulated when the female is in contact with the saliva of a sexually mature boar. Androstenol is one of the pheromones produced in the submaxillary salivary glands of boars that will trigger the female's response.[42] The female cervix contains a series of five interdigitating pads, or folds, that will hold the boar's corkscrew-shaped penis during copulation.[43] Females have bicornuate uteruses and two conceptuses must be present in both uterine horns for pregnancy to be established.[44] Maternal recognition of pregnancy in pigs occurs on days 11 to 12 of the pregnancy.[45]

In many ways, their behaviour appears to be intermediate between that of other artiodactyls and of carnivores.[46] Domestic pigs seek out the company of other pigs, and often huddle to maintain physical contact, although they do not naturally form large herds. They typically live in groups of about 8-10 adult sows, some young individuals, and some single males.[47]

Because of their relative lack of sweat glands, pigs often control their body temperature using behavioural thermoregulation. Wallowing, which often consists of coating the body with mud, is a behaviour frequently exhibited by pigs.[48] They do not submerge completely under the mud, but vary the depth and duration of wallowing depending on environmental conditions.[48] Typically, adult pigs start wallowing once the ambient temperature is around 17-21 °C. They cover themselves from head to toe in mud.[48] Pigs may use mud as a sunscreen, or as a method of keeping parasites away.[48] Most bristled pigs will “blow their coat”, meaning that they shed most of the longer, more-coarse stiff hair once a year, usually in Spring or early Summer, to prepare for the warmer months ahead.[49]

If conditions permit, domestic pigs feed continuously for many hours and then sleep for many hours, in contrast to ruminants which tend to feed for a short time and then sleep for a short time. Pigs are omnivorous, and are highly versatile in their feeding behaviour. As they are foraging animals, they primarily eat leaves, roots, fruits and flowers.[50] Domestic pigs are highly intelligent animals,[51] and according to David DiSalvo's writing in Forbes, they are "widely considered the smartest domesticated animal in the world. Pigs can move a cursor on a video screen with their snouts and even learn to distinguish between the scribbles they knew from those they saw for the first time."[52][a][56][57]

“Rooting” is an instinctual behavior in pigs that is characterized by a pig nudging its snout into something. Similar to a cat’s kneading, rooting is found comforting, first happens when they are born in order to obtain their mother’s milk, and can become a habitual, obsessive behavior which is most prominent in piglets weaned too early. Often, pigs will root and dig into the ground to forage for food. Rooting is known to also be used as a means of communication.[58]

A behavioural characteristic of domestic pigs which they share with carnivores is nest-building. Sows root in the ground to create depressions and then build nests in which to give birth. First, the sow digs a depression about the size of her body. She then collects twigs, grasses and leaves, and carries these in her mouth to the depression, building them into a mound. She distributes the softer, finer material to the centre of the mound using her feet. When the mound reaches the desired height, she places large branches, up to 2 metres in length, on the surface. She enters into the mound and roots around to create a depression within the gathered material. She then gives birth in a lying position, which, again, is different from other artiodactyls, which usually give birth in a standing position.[46]

Nest-building behaviour is an important part in the process of pre and post-partum maternal behaviour. Nest-building will occur during the last 24 hours before the onset of farrowing, and becomes most intense during 12 to 6 hours before farrowing.[59] Nest-building is divided into two phases: one of which is the initial phase of rooting in ground while the second phase is the collecting, carrying and arranging of the nest material.[59] The sow will separate from the group and seek a suitable nest site with some shelter from rain and wind that has well-drained soil. This nest-building behaviour is performed to provide the offspring with shelter, comfort, and thermoregulation. The nest will provide protection against weather and predators, while keeping the piglets close to the sow and away from the rest of the herd. This ensures they do not get trampled on and that other piglets are not stealing milk from the sow.[60] Nest-building can be influenced by internal and external stimuli. Internal hormonal changes and the completion of one nesting phase are indicators of this maternal behaviour.[60] The onset is triggered by the rise in prolactin levels, which is caused by a decrease in progesterone and an increase in prostaglandin, while the gathering of the nest material seems to be regulated more by external stimuli such as temperature.[59] The longer time spent on nest-building will increase pre-partum oxytocin.

Compared to most other mammals, pigs display complex nursing and suckling behaviour.[61] Nursing occurs every 50–60 minutes, and the sow requires stimulation from piglets before milk let-down. Sensory inputs (vocalisation, odours from mammary and birth fluids and hair patterns of the sow) are particularly important immediately post-birth to facilitate teat location by the piglets.[62] Initially, the piglets compete for position at the udder, then each piglet massages around its respective teat with its snout, during which time the sow grunts at slow, regular intervals. Each series of grunts varies in frequency, tone and magnitude, indicating the stages of nursing to the piglets.[63]

The phase of competition for teats and of nosing the udder lasts for about one minute, and ends when milk flow begins. In the third phase, the piglets hold the teats in their mouths and suck with slow mouth movements (one per second), and the rate of the sow’s grunting increases for approximately 20 seconds. The grunt peak in the third phase of suckling does not coincide with milk ejection, but rather the release of oxytocin from the pituitary into the bloodstream.[64] Phase four coincides with the period of main milk flow (10–20 seconds) when the piglets suddenly withdraw slightly from the udder and start sucking with rapid mouth movements of about three per second. The sow grunts rapidly, lower in tone and often in quick runs of three or four, during this phase. Finally, the flow stops and so does the grunting of the sow. The piglets may then dart from teat to teat and recommence suckling with slow movements, or nosing the udder. Piglets massage and suckle the sow’s teats after milk flow ceases as a way of letting the sow know their nutritional status. This helps her to regulate the amount of milk released from that teat in future sucklings. The more intense the post-feed massaging of a teat, the greater the future milk release from that teat will be.[65]

In pigs, dominance hierarchies can be formed at a very early age. Domestic piglets are highly precocious and within minutes of being born, or sometimes seconds, will attempt to suckle. The piglets are born with sharp teeth and fight to develop a teat order as the anterior teats produce a greater quantity of milk. Once established, this teat order remains stable with each piglet tending to feed from a particular teat or group of teats.[46] Stimulation of the anterior teats appears to be important in causing milk letdown,[66] so it might be advantageous to the entire litter to have these teats occupied by healthy piglets. Using an artificial sow to rear groups of piglets, recognition of a teat in a particular area of the udder depended initially on visual orientation by means of reference points on the udder to find the area, and then the olfactory sense for the more accurate search within that area.[67]

Pigs have panoramic vision of approximately 310° and binocular vision of 35° to 50°. It is thought they have no eye accommodation.[68] Other animals that have no accommodation, e.g. sheep, lift their heads to see distant objects.[69] The extent to which pigs have colour vision is still a source of some debate; however, the presence of cone cells in the retina with two distinct wavelength sensitivities (blue and green) suggests that at least some colour vision is present.[70]

Pigs have a well-developed sense of smell, and use is made of this in Europe where they are trained to locate underground truffles. Olfactory rather than visual stimuli are used in the identification of other pigs.[71] Hearing is also well developed, and localisation of sounds is made by moving the head. Pigs use auditory stimuli extensively as a means of communication in all social activities.[72] Alarm or aversive stimuli are transmitted to other pigs not only by auditory cues but also by pheromones.[73] Similarly, recognition between the sow and her piglets is by olfactory and vocal cues.[74]

The use of pig milk for human consumption does take place, but as there are certain difficulties in obtaining it, there is little commercial production.

Livestock pigs are exhibited at agricultural shows, judged either as stud stock compared to the standard features of each pig breed, or in commercial classes where the animals are judged primarily on their suitability for slaughter to provide premium meat.

In some developing and developed nations, the domestic pig is usually raised outdoors in yards or fields. In some areas, pigs are allowed to forage in woods where they may be taken care of by swineherds. In industrialized nations such as the United States, domestic pig farming has switched from the traditional pig farm to large-scale intensive pig farms. This has resulted in lower production costs, but can cause significant cruelty problems, and demand for pasture-raised pork in the United States has increased as consumers become concerned with humane treatment of livestock.[76]

They were soon crossbred with a variety of other smaller breeds, with seperate locations breeding different lineages. These miniature pigs soon gained attention, even more so than the original Vietnamese Pot-bellied. As a result, many pet pigs are now of unknown genetic descent.

Domestic pigs are highly intelligent, social and emotional creatures. They are considered hypoallergenic, and are known to do quite well with people who have usual animal allergies. Since these animals are known to have a life expectancy of 15 to 20 years, they require long-term commitment.

Un-neutered male and female swine may express unwanted, aggressive behavior, and are prone to developing serious health issues.[77] Regular trimming of the hooves is necessary; hooves left untreated cause major pain in the pig, can create malformations in bone structure, and may cause it to be more susceptible to fungal growth between crevices of the hoof.[78] As prey animals, natural behavior causes pigs to have a fear of being picked up, but they will usually calm down once placed back onto the ground. When holding the pig, supporting it under the legs is not as stressful for the animal.[79] Pigs need enrichment activities to keep their intelligent minds occupied; if pigs get bored, they often get destructive.[80] As rooting is found to be comforting, pigs kept in the house may root in household objects, and as such, should have a box with rocks, soil, straw, and/or other material for them to root in instead.

The domestic pig, both as a live animal and source of post-mortem tissues, is one of the most valuable animal models used in biomedical research today, because of its biological, physiological and anatomical similarities to human beings.[81][82] For instance, human skin is very similar to pig skin, therefore pig skin has been used in many preclinical studies.[81][82] Porcine are used in finding treatments, cures for diseases, xenotransplantation, and for general education. They are also used in the development of medical instruments and devices, surgical techniques and instrumentation, and FDA-approved research. As part of animal conservation (The Three Rs (animals)), these animals contribute to the reduction methods for animal research, as they supply more information from fewer animals used, for a lower cost.

Pigs are currently thought to be the best non-human candidates for organ donation to humans. The risk of cross-species disease transmission is decreased because of their increased phylogenetic distance from humans.[83] They are readily available, their organs are anatomically comparable in size, and new infectious agents are less likely since they have been in close contact with humans through domestication for many generations.[84]

To date, no xenotransplantation trials have been entirely successful due to obstacles arising from the response of the recipient’s immune system—generally more extreme than in allotransplantations, ultimately results in rejection of the xenograft, and in some cases result in the death of the recipient—including hyperacute rejection, acute vascular rejection, cellular rejection and chronic rejection. An early major breakthrough was the 1,3 galactosyl transferase gene knockout.[85]

Examples of viruses carried by pigs include porcine herpesvirus, rotavirus, parvovirus, and circovirus. Of particular concern are PERVS (porcine endogenous retroviruses), vertically transmitted microbes that embed in swine genomes. The risks with xenosis are twofold, as not only could the individual become infected, but a novel infection could initiate an epidemic in the human population. Because of this risk, the FDA has suggested any recipients of xenotransplants shall be closely monitored for the remainder of their life, and quarantined if they show signs of xenosis.[86]

Pig cells have been engineered to inactivate all 62 PERVs in the genome using CRISPR Cas9 genome editing technology, and eliminated infection from the pig to human cells in culture.[87]

^Fraser, D., (1973). The nursing and suckling behaviour in pigs. I. The importance of stimulation of the anterior teats. British Veterinary Journal, 129: 324-336

^Jeppesen, L.E. (1982). "Teat-order in groups of piglets reared on an artificial sow. II. Maintenance of teatorder with some evidence for the use of odour cues". Applied Animal Ethology. 8: 347–355. doi:10.1016/0304-3762(82)90067-0.

^ abHerron, Alan J. (5 December 2009). "Pigs as Dermatologic Models of Human Skin Disease"(PDF). ivis.org. DVM Center for Comparative Medicine and Department of Pathology Baylor College of Medicine Houston, Texas. Retrieved 27 January 2018. pig skin has been shown to be the most similar to human skin. Pig skin is structurally similar to human epidermal thickness and dermal-epidermal thickness ratios. Pigs and humans have similar hair follicle and blood vessel patterns in the skin. Biochemically pigs contain dermal collagen and elastic content that is more similar to humans than other laboratory animals. Finally pigs have similar physical and molecular responses to various growth factors.

^David DiSalvo's article in Forbes refers to via an article in Penn State Agricultural Magazine[53] referenced from 'Pork' by Catherine Becker at The Ohio State University[54] referencing work by Candace Croney, now head of Purdue center for animal welfare science[55]